Multimedia content transmission networks may be used in particular for pay-per-view television applications.
In transmission networks of this type, multimedia content are transmitted in encrypted form and can be reproduced by a receiver terminal only subject to certain conditions. Access to such multimedia content is generally controlled as a function of access entitlements and access criteria.
In such a context, a terminal receives its entitlements via a unique message that is sent to it individually. Such a message may be an initial entitlements message of the Entitlement Management Message (EMM) type. A message of this type is a personalized message that can be sent to a single subscriber or to a group of subscribers.
Then each terminal of the transmission network may optionally access certain digital content, as a function of its particular entitlements. Firstly, the digital content is transmitted in encrypted form using an encryption key (or control word (CW)) and, secondly, the encryption key is transmitted in a message synchronized with transmission of the digital content. Thus in order to access the digital content it is first necessary to decrypt the encryption key associated with it and that is received in an access control message, for example a message of the Entitlement Control Message (ECM) type, synchronized with a data stream transporting the digital content. This first decryption may be effected if the terminal already has in its possession an operating key enabling it to decrypt the digital content. This operating key is included in the entitlements that the terminal has received via the EMM-type message. Consequently, if the entitlements of the terminal authorize it to decrypt the encryption key received in association with the digital content using the operating key, it is then in a position to decrypt the digital content.
In the above context, access control is based on access entitlements assigned to each terminal.
Moreover, for some applications, or in some access control systems, in addition to these access entitlements, there is also provision for controlling access to a digital content on the basis of access criteria. Satisfying those access criteria may, for example, correspond to a specific subscription being present and valid or an account of the user of the terminal being sufficiently in credit to be able to pay for access to content the cost of which, or the amount to be debited, constitutes the access criterion.
Thus a terminal may be authorized to access a digital content if firstly its entitlements enable it to decrypt the received digital content and secondly the criteria for access to that digital content are satisfied. These access criteria are transmitted in messages of the ECM type synchronously with the corresponding data stream. This synchronization is required in a one-to-many communications context, i.e., communication from one source to a plurality of receivers, for example in broadcast or multicast mode.
In this context, the access entitlements transmitted in the EMM messages may be individualized for each subscriber while the access criteria transmitted in ECM messages in general broadcast mode are common to all subscribers.
FIG. 1 shows a prior art system for broadcasting digital content with controlled access.
Such a system includes one or more access entitlement management entities 11, a digital content transmission entity 14, and terminals 12 and 15. The entitlements management entity 11 sends the terminal 12 an EMM initial entitlements message 101 to send it its own entitlements A and sends the terminal 15 an EMM initial entitlements message 102 to send it its own entitlements B.
The transmission entity 14 then uses the general broadcast mode to broadcast (104) a digital content to the terminals 12 and 15 of the system. Synchronously with this general broadcast, entitlement control messages (ECM) 110 are transmitted in the same general broadcast. These messages indicate access criteria common to all the general broadcast destination terminals.